yuvald-sweet-security wrote:
@yonghong-song thank you for taking the time to help with this issue and
providing your suggestions, it’s greatly appreciated. I am also glad to hear
that asm barriers are no longer necessary, as they caused quite some trouble
for me in the past. However, I've encountered a few issues while attempting to
apply this as a solution to the problem with moving to mpcu v3.
* while you are correct in that removing the barriers fixes the verifier
failure in `trace_ret_vfs_writev_tail`, the original issue I've pointed out -
the increased size of kernel verifier states and instructions when using v3 -
still remains:
```
$ sudo /root/veristat /tmp/test-v1.o --filter=trace_ret_vfs_writev_tail
Processing 'test-v1.o'...
File Program Verdict Duration (us) Insns States
Program size Jited size
--------- ------------------------- ------- ------------- ----- ------
------------ ----------
test-v1.o trace_ret_vfs_writev_tail success 23158 25292 1874
7163 38486
--------- ------------------------- ------- ------------- ----- ------
------------ ----------
Done. Processed 1 files, 0 programs. Skipped 1 files, 158 programs.
$ sudo /root/veristat /tmp/test-v3-nobarrier.o
--filter=trace_ret_vfs_writev_tail
Processing 'test-v3-nobarrier.o'...
File Program Verdict Duration (us) Insns
States Program size Jited size
------------------- ------------------------- ------- ------------- ------
------ ------------ ----------
test-v3-nobarrier.o trace_ret_vfs_writev_tail success 69161 104971
7618 6999 37490
------------------- ------------------------- ------- ------------- ------
------ ------------ ----------
Done. Processed 1 files, 0 programs. Skipped 1 files, 158 programs.
```
As you can see the v3 codegen causes the verifier to consume about 4 times the
number of instructions when verifying it, and while it's not such a big issue
for this particular function, it can be an issue for larger functions which are
already close to the verifier's 1 million instructions limit as this can cause
them to go over it.
* Many verifier failures still remain after removing the barriers. e.g. the
function `vfs_writev_magic_return` still fails even after I've removed all asm
volatile/barrier vars (you can use
[patch.txt](https://github.com/user-attachments/files/19407900/patch.txt) which
removes all of them). Also, in some of the code that I have removing the
barrier vars on v3 codegen actually introduces more verifier failures (compared
to v3 with barrier vars), I'll see if I can make a minimal example of this.
thank you again for your thoughtful advice and support
https://github.com/llvm/llvm-project/pull/107008
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